The internal pressure of a sealed battery sometimes increases abnormally depending on the conditions of use. For example, nonaqueous electrolyte secondary batteries, represented by a lithium ion secondary battery, have a character such that their internal pressure increases abnormally when overcharge occurs and the internal pressure also increases when overcurrent occurs due to a short-circuit. If the internal pressure of the battery increases abnormally causing an outer can of the battery to explode, electronic equipment containing the battery is damaged. Furthermore, leakage of corrosive gas or an electrolyte from the exploded outer can has adversely affects corrosion of the electronic equipment. In order to avoid these drawbacks, an explosion-proof sealed battery has a system to prevent an abnormal increase of the internal pressure. For example, the abnormal increase of the internal pressure of the battery in case of the overcharge can be prevented by breaking current in the battery. This is because chemical reactions do not occur in the battery when the current breaks.
As systems to prevent the abnormal increase of the internal pressure, JP-A-6-140011, JP-A-11-86822, JP-A-6-338305, and JP-A-8-153510 disclose sealed battery terminals including an electrically-conductive flexible valve deformable by a battery pressure increase in a positive or a negative electrode terminal cap and a valve support electrically connected to the valves. In these safety valve systems, when the battery internal pressure increases a little, the electrical connection between the valve and the valve support is broken by valve deformation, and when the battery internal pressure increases much more to increase the valve deformation, the valve is broken to break the electrical connection between the valve and the valve support permanently.
The structure of the sealed battery terminal disclosed in JP-A-8-153510 will be explained with reference to FIG. 7. FIG. 7 is a longitudinal sectional view showing the sealed battery terminal disclosed in JP-A-8-153510.
The sealed battery terminal 50 includes a metal cap terminal 51, a flexible metal rupture disk 52 placed under the cap terminal 51 and the rupture disk 52 being deformed with increase of battery internal pressure, an insulating ring 53 placed under the rupture disk 52, and a punched metal plate 54 placed under the insulating ring 53 and the plate 54 having at least a hole at a central portion of the plate. A strip-shaped terminal plate 56 made of a ribbon metal plate bended convexly is crimped to the punched metal plate 54 interposing an insulating plate 55, and an opening part 57 is arranged at a convex part 56a of the strip-shaped terminal plate 56. The convex part 56a is inserted into a center hole of the punched metal plate 54, and a top surface of the convex part 56a is partially welded to the rupture disk 52 to form a welded part 58. Accordingly, the rupture disk 52 and the strip-shaped terminal plate 56 are electrically connected with each other through the welded part 58. The welded part 58 has a ring shape with a circular non-welded part corresponding to the opening part 57, and when a battery internal pressure reaches a predetermined value, the welded part 58 is broken by a stress deforming a central portion of the rupture disk 52 toward outside to break the electrical connection between the rupture disk 52 and the strip-shaped terminal plate 56.
In the sealed battery terminal 50 of the related-art example described above, since the welded part 58 is formed between the rupture disk 52 and the convex part 56a of the strip-shaped terminal plate 56, the working pressure can be controlled by changing not only the thickness of the flexible metal rupture disk 52 but also the welded area of the welded part 58. Accordingly, the sealed battery terminal 50 of the related-art example described above has an advantageous effect that a sealed battery terminal with less variation and high reliability is obtained, since even when an increase of the battery internal pressure due to short circuit, overcharge, reverse-charge or the like of the battery deforms the rupture disk 52 to break the welded part 58, it does not occur that a central portion of the welded part 58 does not break completely to maintain abutment between the safety rupture disk 58 and the convex part 56a of the strip-shaped terminal plate 56.
In the sealed battery terminal 50 disclosed in JP-A-8-153510, the welded part 58 is subjected to full penetration welding, since the rupture disk 52 and the convex part 56a of the strip-shaped terminal plate 56 are lap-welded. This also applies to the sealed battery terminal disclosed in JP-A-6-338305 is same. Furthermore, JP-A-6-338305 and JP-A-8-153510 disclose that as a forming method of a welded part of a sealed battery terminal, ultrasonic welding or laser welding can be used so as to set various welding conditions easily.
Since ultrasonic welding causes stress to the welding member, it is hard to use, especially when the welding member has a partially thin notch part. On the other hand, in laser welding, since the irradiation area is small, even a welding member with a partial notch is no trouble. However, if a welded part has been subjected to full penetration welding as mentioned above, laser welding has the problem that the laser irradiation conditions are quite limited and lack productivity, and due to variation of the thickness, size or the like of a welding member, the welding may be too weak, or too strong to stave the welding member. However, in order to improve manufacturing efficiency, it is required to adopt a welding means using a high energy beam such as a laser beam and an electron beam that can be emitted away from the welding member rather than the ultrasonic welding in which the welding means needs a direct contact to the welding member.